In the 70s, the American designer Halston was one of fashion’s biggest stars. But since his death from an Aids-related illness in 1990, his reputation has dimmed, despite attempts to revive his company (one of them involving Harvey Weinstein and Sarah Jessica Parker). Now comes this flattering, myth-inflating documentary by Frédéric Tcheng, who gives us the story of Roy Halston Frowick, a kid from Des Moines, Iowa, who reinvented himself in New York as a milliner to the super-rich and joined the big league by putting Jackie Kennedy in a pillbox hat at JFK’s inauguration. He became one-name famous by designing party dresses that one interviewee gigglingly describes as best worn without knickers.

This profile has a pretentious – and pointless – framing device in which fashion writer Tavi Gevinson plays a fictional archivist who turns detective to investigate his life. As well as understanding the fashion mood of the 70s, Halston savvily realised that dressing his beautiful celebrity pals Liza Minnelli, Anjelica Huston and Bianca Jagger was the best possible advertisement. He also increased diversity on the runway, promoting African American models, and his muse was Pat Ast, the frizzy-haired size-20 actor who had worked with Andy Warhol.

His secretary pinpoints the beginning of the end for Halston as the opening of the Studio 54 nightclub, where he picked up a $1,000-a-week cocaine habit. He spent money like crazy, once sending a private plane to fetch his dinner. Then there were the business decisions. In 1983, he signed a disastrous $1bn deal with the US high-street chain JC Penney – appalling his elite fanbase. By 1984, Halston lost the rights to his name and company.

The archive clips suggest Halston is a role Richard E Grant was born to play: the designer had a long-limbed loucheness, grandiose affectations and put-on accent, along with a fierce perfectionism.